The present invention relates in general to methods for pressing and forming powders to solid objects, and in particular to a new and useful process for producing a porous part having opposite concave and convex surfaces.
In order to be able to produce cathodes for travelling wave tubes which heat up quickly, the cathode mass must be as small as possible.
A reduction of the cathode mass however, is only possible by way of the thickness of the cathode pill, if the current density is predetermined (current per emission surface). In order to meet electron-optical requirements, the emitting surface has to be concave.
Another important requirement is uniform electron emission over the entire cathode surface, which results in the requirement of homogeneous porosity of the matrix element.
Attempts are made to meet these requirements by pressing metal powder (as a rule, tungsten or tungsten mixed with Os, Ir, Ru, Re) as cylindrical, plane-parallel elements with sufficient thickness in a so-called cathode holder, sintering them at a high temperature, and subsequently producing the concave shape by mechanical processing. This direct pressing of the powder in the cathode holder has the advantage, as compared with processes in which cathode pills are produced without a cathode holder, that the holder element necessary for attachement in the tube system is already rigidly connected to the cathode after sintering, and that additional processes, such as soldering or welding, for example, are not necessary. By pressing plane-parallel elements, sufficiently homogeneous porosity distribution is guaranteed.
Both processes (with or without the cathode holder) have the disadvantages, however, that production of the concave shape is complicated and expensive (in particular when the concave shape is deep) and that the cathode mass becomes significantly greater than necessary, since essentially, only a concave-convex disk with a constant thickness is needed.
If an attempt is made to produce the concave cathode surface right away, during pressing, by giving the top die a convex shape, then non-homogeneous porosity distributions are produced, particularly in the case of deep concave shapes, because of non-homogeneous compression of the metal powders.
If an attempt is made, in addition to this, to reduce the cathode mass, by giving the cathode holer bottom a concave shape, then the non-homogeneous porosity distribution is even further increased.
If pressing is carried out without a cathode holder, extremely difficult and time-consuming processing of the spherical surfaces on both sides must be carried out; this becomes critical in particular when one considers that porous pills with thicknesses from 150 to 300 .mu.m are desired.
In addition, there is the difficult problem of attachment of the cathode pill and suppression of lateral and rearward electron emission, and evaporation of emission material.